What ALT and AST Stand For
ALT stands for alanine aminotransferase (also called alanine transaminase), and AST stands for aspartate aminotransferase (also called aspartate transaminase) 1.
Enzyme Definitions and Characteristics
ALT (Alanine Aminotransferase)
- An enzyme highly concentrated in the liver with minimal presence in skeletal muscle and kidney
- More specific for liver injury due to its predominant hepatic localization 1
- Normal reference ranges: 29-33 IU/L in men and 19-25 IU/L in women 1
AST (Aspartate Aminotransferase)
- An enzyme concentrated in the liver but also present in heart, skeletal muscle, kidneys, brain, and red blood cells
- Less specific for liver injury because elevations can occur from non-hepatic sources 1
- Generally has higher baseline levels than ALT 2
Clinical Significance
Both aminotransferases are biomarkers of hepatocyte damage rather than actual measurements of liver function 1. When hepatocytes are injured through necrosis, inflammation, toxic injury, or ischemia, these intracellular enzymes leak into the bloodstream, causing elevated serum levels 1.
Key Clinical Points:
- Hepatocellular injury pattern: Aminotransferases elevated disproportionately higher than alkaline phosphatase 1
- Severity classification 1:
- Mild: <5 times upper reference limit
- Moderate: 5-10 times upper reference limit
- Severe: >10 times upper reference limit
Important Caveat:
Despite being called "liver function tests," ALT and AST do not measure actual liver synthetic function—albumin and prothrombin time are the true functional markers 1. The term "liver function tests" is a misnomer when referring to aminotransferases 2.